The present invention relates to an electrically conductive coating, in particular for toner transfer drums, which is producible by hydrolytic condensation of a mixture comprising at least one hydrolysable silicon compound of general formula:SiXnR(4-n)  (I),wherein the radicals X, which can be the same or different, and can in particular be selected from the group comprising optionally alkoxy groups substituted by halogen or alkoxy, hydroxy groups. halogen, aryloxy and acyloxy groups, hydrogen, straight-chained, branched or cyclic alkyl, alkenyl, and alkinyl radicals. In particular with 5 to 20 carbon atoms, wherein the radicals R, which may be the same or different, are selected from the group comprising alkyl, alkenyl, alkinyl, aryl, whereby the groups can optionally have one or more substitutes inert under reaction conditions, and the abovementioned alkyl radicals include cyclic and aryl-substituted radicals, the alkenyl and alkinyl groups can likewise be cyclic and the aryl groups also include alkaryl groups, whereby the abovementioned groups optionally have one or more substitutes inert under reaction conditions and whereby n is a whole number from 1 to 4, whereby the mixture also includes one or more hydrolytically condensable compounds of silicon or other elements selected from the group comprising B, AL, P, Sn, Pb, transition metals, lanthanides and actinides, whereby at least one of the monomer compounds to be used for condensation is derived from a silicon compound of general formula:ZaSiYbR(4-a-b)  (II)wherein the radicals and indices are the same or different and have the following meaning:    Y=hydrogen, halogen, hydroxy, optionally substituted alkoxy, acyloxy, alkyl carbonyl, alkoxy carbonyl, or NR′2;    Z=quaternary ammonium salts derived from R22N—(R3—NR2—)k—R3, or    Z=1/m Am⊖R22—N⊕—R3—, 1/m Am⊖(HO—R3)2N⊕—R3—, 1/m Am⊖(R2O)2P⊕(O)—R3—, 1/m Am⊖R22—N)2C—S⊕—R3, 3/n Kn⊕(OOC⊖)2—N—R3—N(COO⊖)—R3—, 1/m Am⊖H2C═C—(R2)—R3 N ⊕ (R22)—R3—, 1/m Am⊖H2C═C—(R2)—CO—O—R3N⊕ (R22)—R3—,    R=optionally substituted alkyl, alkenyl, aryl, alkylaryl, or arylalkyl;    R′=hydrogen, alkyl, or aryl;    R2=hydrogen, optionally substituted alkyl, alkenyl, aryl, alkylaryl, or arylalkyl;    R3=optionally substituted alkylene, alkenylene, or arylene;    Am⊖=anion with a valence of m;    Kn⊕=cation with a valence of n;    a=1, 2, or 3;    b=1, 2, or 3;    a+b=2, 3, or 4; and    k=0, 1, 2, or 3. An antistatic coating of the previous type, in particular for toner transfer drums, is disclosed in DE 199 42 423 A1.
Toner transfer drums for laser printers, copiers, and fax machines are especially favourably produced in various dimensions and unit quantities by using organic photoconductors, whereby, by the incorporation of photosensitive pigments in specially purified polymers in the form of a so-called generator layer on an aluminium support and a transport layer thereon composed of a polymer doped with said photoconductor, the electrophotographic function is achieved. Although the above transport layer is mostly comprised of polycarbonate and stable, a relatively soft layer with minimal scratch resistance results from depositing the photoconductors designated as OPC's.
It is evident that the large number of different technical solutions, the result of which ought to be improved abrasion resistance of such systems, cannot at present yet be manufactured in a marketable product. Of the existing proposals, as to how hardness and friction resistance of such systems for technical application can be improved, the application of silane hydrolysate enamels has proven particularly successful on account of their extremely dense network structure or those hybrid polymers formed partially from inorganic constituents. At the same time it is evident that a combination of high friction resistance with the required conductibility was previously always associated with skimming with another property.
For example, Patent DE 30 32 773 describes an image carrier having a hard, cross-linked surface layer, which however exhibits too high a surface resistance. An image carrier having a surface layer based on metal alkoxide and alkyl/aryltrialkoxysilane, described in Patent DE 40 10 328, has the same properties, whereby it becomes clear that, based on their extraordinarily dense network structure, silane hydrolysate lacquers or those hybrid polymers formed partially from inorganic constituents are particularly significant for the coating of image carriers or toner transfer drums. EP 0 450 625 A1 describes silane hydrolysate enamels which are quick-setting under UV radiation and based on vinyl mercapto addition for producing scratch-resistant coatings. These coating systems harden in the absence of a photo initiator within a brief curing period.
As based on DE 199 42 423 A1, this can also be used for toner transfer drums, when specific electrolytically effective compounds such as quaternary ammonium salts are incorporated for diverting charges at illuminated points. Unfortunately, such friction-resistant coatings have a high polar proportion of surface energy, such that low-ohm toner constituents are deposited, and after relatively few printings an unwanted image run ensues. In search of the causes for this behaviour it was assumed that if possible non-condensed silanol groups determine the polar portion of the layer. Using Si29 spectrography the content of varying degrees of condensation can be determined as compared to ascertaining the silanol with the proton resonance spectroscopy with adequate accuracy. Checking the silanol content can establish that only a minimal silanol portion can be further condensed by relatively long subsequent thermal curing at 80° C. This corresponds to still occurring insufficient imaging of dots at 600 dpi single printing.
A feature of the present invention is to provide an antistatic coating of the type initially described, in particular an electrically conductive lacquer, after curing at low temperatures, for the coating of toner transfer drums, whereby the resulting coating has high scratch resistance and good adhesion to the substrate. A further requirement is that said coating should be satisfactorily curable in the shortest possible time at relatively low temperatures. In addition, the conductive coating material should have sufficient storage stability in order to enable conversion in practice. To enable unrestricted use of the corresponding finished products (copiers, printers, et al.) considerable insensitivity to tropical climates (high humidity) should be present.
For this, the existing technical deficits with respect to deposits, which originate from the toner, should be hindered by a specific setting of the polar portion of the surface energy. In a ‘normal’ layer these are constantly being rubbed off with the paper, and are concentrated at high friction resistance and on account of the high adhesion forces, based on the polar portion, on such a scratch-resistant layer. Accordingly, it is a further feature of the invention to minimise the polar portion, without excessively severely restricting the electrical properties.